(i) Field of the Invention
The present invention relates to a combustible gas sensor for detecting a combustible gas contained in a gas to be measured. The present invention also relates to a method for detecting the deterioration of a catalyst for cleaning the exhaust gas of an automobile.
(ii) Description of the Related Art
A combustible gas sensor intends to detect a combustible gas contained in a gas to be measured, such as a gas generated at combustion or an exhaust gas, and it has been required to quantitatively determine the concentration of the combustible gas by the sensor. The exhaust gas is exhausted from, for example, an internal combustion engine, an external combustion engine or a combustion furnace which utilizes heavy oil, gas oil, gasoline, a natural gas or the like as a fuel.
In the combustible gas sensor, the so-called contact combustion type is employed, and in this type, the combustible gas is burned with the aid of a platinum catalyst and a temperature raised by the combustion heat is then measured. In a conventional contact combustion type combustible gas sensor, an oxidation catalyst of a noble metal such as Pt, Pd or Rh is supported on beads formed by sintering alumina in a porous state on a platinum coil. The platinum coil is heated by an external heater up to about 300xc2x0 C. and then brought into contact with the gas to be measured, so that combustion occurs to raise the temperature of the platinum coil. The thus raised temperature is then detected as a change of the electric resistance of the platinum wire buried in the beads by means of a bridge circuit.
The conventional combustible gas sensor, however, is constituted so that the combustible gas may come in contact with the platinum resistor, and therefore when the sensor is used at a temperature as high as 900xc2x0 C. the resistance value of the platinum resistor changes inconveniently. Thus, the conventional combustible gas sensor has a drawback that its use at a high temperature is impossible. In addition, when the temperature of the gas to be measured changes remarkably, for instance, when it changes from room temperature to about 900xc2x0 C. there is no way to accurately measure the temperature rise attributable to the heat generation of the combustible gas contained therein.
On the other hand, various combustible gas sensors using oxide semiconductors have also been investigated. However, the oxide semiconductor type combustible gas sensor has a disadvantage that its performance is vitally affected by oxygen, humidity or the like, and another disadvantage that the resistance value of the semiconductor changes at a high temperature.
Heretofore, there has been researched a method for detecting the deterioration of a catalyst such as an exhaust gas cleaning catalyst which intends to eliminate the combustible gas. For instance, Japanese Utility Model Application Laid-open No. 61919/1987 has suggested a method for detecting the deterioration of a catalyst which comprises arranging temperature sensors on the upstream side and on the downstream side of the catalyst, respectively, and then comparing a temperature of the exhaust gas on the upstream side with that of the exhaust gas on the downstream side. In this method, the deterioration of the catalyst is required to be judged after an automobile has run at a constant speed of 40 to 60 km/hr for several minutes. This constant running is necessary to thermally stabilize an exhaust system, because the catalyst has a large heat capacity. Thus, in order to improve the detection accuracy of the catalyst deterioration, the automobile is required to run at the constant speed for a further long time.
However, under actual running conditions of the automobile where acceleration and deceleration are repeated, the above-mentioned requirement of the running at the constant speed can hardly be met, and therefore it is difficult to detect the deterioration of the catalyst with a high precision. Furthermore, in this method, a sufficient temperature difference cannot be obtained unless the temperature sensors are inserted into an exhaust tube so that they may be close to the central axis of the exhaust tube, and hence this method has a drawback that the pressure of the exhaust gas is increased and the output power of the engine is consequently reduced. In addition, the two temperature sensors are required, which makes the system complicated, with the result that cost increases inconveniently.
When the combustible gas sensor is used in the exhaust system of the automobile, the output of the sensor element is connected to an electronic device other than the sensor element, a central processing unit or the like, and the temperature or the like is detected by the electronic device, the central processing unit or the like. Here, a resistance value R of a resistor having a positive resistance temperature coefficient is represented by R=R0(1+atxe2x88x92xcex2t2) wherein t is a temperature, and R0 is a resistance value of the resistor at 0xc2x0 C. but it is not limited to the resistance value at 0xc2x0 C. and it may be the definite resistance value of the resistor at a certain temperature.
Accordingly, in order that the electronic device, the central processing unit or the like measures the temperature or the like, it is necessary that the resistance value at the certain temperature of the resistor in the specific sensor element connected to the electronic device or the like should be previously input to the electronic device or the like. Furthermore, when the sensor elements are attached to the automobiles on such a mass production line as in an assembly plant of the automobiles, the resistance value of the resistor in each sensor element is required to be promptly put to the electronic device or the like. For example, it is impractical that the resistance value of the resistor is input to the computer of each automobile by way of a keyboard.
However, in the process of manufacturing the sensor elements, the resistance value of the resistor may inevitably scatter to some extent. For example, the resistor can often be formed by printing its pattern on the surface of a ceramic green sheet and then baking the green sheet having the resistor pattern. The resistance value of the thus formed resistors typically has a deviation of xc2x110%.
Japanese Patent Application Laid-open No. 279831/1992 has described a technique of trimming the resistor by means of laser irradiation in order to minimize the deviation in the resistance values of the resistor. However, when the resistor is trimmed by the laser irradiation, the temperature of the resistor rises. Since the resistor has a large resistance temperature coefficient, it is difficult to heighten the precision of the resistance value of the resistor, so that the resistance value of each sensor element largely scatters sometimes.
An object of the present invention is to provide a combustible gas sensor which is excellent in durability at a high temperature and which can hence measure a gas to be measured even if its temperature noticeably fluctuates.
Another object of the present invention is to provide a method for using the combustible gas sensor, particularly a method for measuring the concentration of a combustible gas.
Still another object of the present invention is to provide a method for detecting, with a high precision, the deterioration of a catalyst which intends to eliminate the combustible gas, even if the temperature of the gas to be measured fluctuates.
According to the first aspect of the invention, there is provided a combustible gas sensor which comprises a base member having a first temperature sensitive portion of a dense ceramic material and a second temperature sensitive portion of a dense ceramic material, a first temperature sensor section and a second temperature sensor section; the first temperature sensor section being provided with the first temperature sensitive portion, a first resistor buried in the first temperature sensitive portion and having a positive resistance temperature coefficient, a first pair of current leads for feeding current to the first resistor and a first pair of voltage leads for detecting the voltage of the first resistor; the second temperature sensor section being provided with the second temperature sensitive portion, a second resistor buried in the second temperature sensitive portion and having a positive resistance temperature coefficient, a second pair of current leads for feeding current to the second resistor, a second pair of voltage leads for detecting the voltage of the second resistor and a porous oxidation catalyst layer which covers at least a part of the surface of the second temperature sensitive portion and which catalyzes the oxidation of a combustible gas.
In the present invention, it is preferred that the first resistor is connected to a first pair of current terminals and a first pair of voltage terminals by way of the first pair of current leads and the first pair of voltage leads, and the second resistor is connected to a second pair of current terminals and a second pair of voltage terminals by way of the second pair of current leads and the second pair of voltage leads.
It is preferred that the first and second temperature sensitive portions have substantially identical shapes and are made of substantially identical materials, and the first and second resistors have substantially identical shapes and are made of substantially identical materials.
Furthermore, a space may be formed between the first temperature sensitive portion and the second temperature sensitive portion, or the space between the first temperature sensitive portion and the second temperature sensitive portion may be filled with the base member.
It is preferred that the first temperature sensitive portion has a first dense ceramic layer covering the first resistor; the second temperature sensitive portion has a second dense ceramic layer covering the second resistor; and the oxidation catalyst layer covers the second dense ceramic layer.
In addition, the base member including the first temperature sensitive portion and the second temperature sensitive portion preferably contains 99% or more of alumina.
Preferably, the oxidation catalyst layer has a cermet layer covering at least a part of the surface of the second temperature sensitive portion; the cermet layer has a skeletal structure containing a ceramic material and a metal which functions as an oxidation catalyst; the skeletal structure is porous; and the particles of the metal are supported on the inner surface of the skeletal structure.
Preferably, at least a part of the surface of the first temperature sensitive portion is covered with a first catalyst layer; at least a part of the surface of the second temperature sensitive portion or the oxidation catalyst layer is covered with a second catalyst layer; and both the first catalyst layer and the second catalyst layer contain a catalyst for oxidizing carbon monoxide.
Preferably, the first temperature sensor section further has a first potentiometric resistor connected in parallel to the first resistor; the resistance of the first potentiometric resistor is regulated by trimming so that an output voltage generated at the feed of a predetermined current to the first resistor may be a predetermined value; the second temperature sensor section further has a second potentiometric resistor connected in parallel to the second resistor; and the resistance of the second potentiometric resistor is regulated by trimming so that an output voltage generated at the feed of a predetermined current to the second resistor may be a predetermined value.
Preferably, the first temperature sensor section further has a first serial resistor connected in series to the first resistor by way of the first pair of voltage leads; the resistance of the first serial resistor is regulated by trimming so that the total of the resistances of the first resistor, the first serial resistor and the first pair of voltage leads may have a certain relation to the resistance of the first resistor; the second temperature sensor section further has a second serial resistor connected in series to the second resistor by way of the second pair of voltage leads; and the resistance of the second serial resistor is regulated by trimming so that the total of the resistances of the second resistor, the second serial resistor and the second pair of voltage leads may have a certain relation to the resistance of the second resistor.
Preferably, the resistance of the first serial resistor is regulated by trimming so that the total of the resistances of the first resistor, the first serial resistor and the first pair of voltage leads may be proportional to the resistance of the first resistor; and the resistance of the second serial resistor is regulated by trimming so that the total of the resistances of the second resistor, the second serial resistor and the second pair of voltage leads may be proportional to the resistance of the second resistor.
The combustible gas sensor of the present invention preferably has a heating/control means for heating and controlling the first resistor or the second resistor to a predetermined temperature. This heating/control means may have a variable power source for applying current or voltage to the first resistor or the second resistor, and the variable power source may regulate the current or the voltage so as to control the first resistor or the second resistor to a predetermined temperature in accordance with the resistance of the first resistor or the second resistor. Alternatively, the heating/control means may have a heater for regulating its output so as to control the first resistor or the second resistor to a predetermined temperature in accordance with the resistance of the first resistor or the second resistor.
According to a second aspect of the present invention, there is provided a method of measuring the concentration of a combustible gas by the use of the above-mentioned combustible gas sensor which comprises a step of applying a current I1 to a first resistor to determine a voltage V1 of the first resistor, a step of applying a current I2 to the second resistor to determine a voltage V2 of the second resistor, and a step of determining a difference between temperatures of the first resistor and the second resistor or a difference between electric power fed to the first resistor and the second resistor on the basis of the current I1, the current I2, the voltage V1 and the voltage V2.
In the present invention, it is preferred that the current I1 is so weak as not to substantially raise the temperature of the first resistor, and the current I2 is so weak as not to substantially raise the temperature of the second resistor.
Furthermore, according to a method for measuring the concentration of a combustible gas by the use of a combustible gas sensor having a heating/control means for heating and controlling the first resistor or the second resistor to a predetermined temperature, the temperature or the resistance of the second resistor may be determined by heating/controlling the first resistor to a predetermined temperature. Alternatively, a difference between electric powers fed to the first resistor and the second resistor may be determined by heating/controlling the first resistor and the second resistor to a predetermined temperature.
According to a third aspect of the present invention, there is provided a method for detecting the deterioration of a catalyst which intends to eliminate a combustible gas by the use of the above-mentioned combustible gas sensor, said method comprising the step of measuring the concentration of the combustible gas contained in a gas to be measured which is discharged through the catalyst, by the combustible gas sensor attached on the downstream side of the catalyst.
In the present invention, output signals of the combustible gas sensor may be accumulated for a predetermined period of time. Alternatively, there may be calculated a product of the output signal of the combustible gas sensor and the flow rate of the gas to be measured. In the case of the latter, the products are preferably accumulated for a predetermined period of time.
According to a fourth aspect of the invention, there is provided a sensor element which comprises a base member having a first temperature sensitive portion of a dense ceramic material and a second temperature sensitive portion of a dense ceramic material, a first temperature sensor section and a second temperature sensor section; the first temperature sensor section being provided with the first temperature sensitive portion, a first resistor buried in the first temperature sensitive portion and having a positive resistance temperature coefficient, a first pair of current leads for feeding current to the first resistor and a first pair of voltage leads for detecting the voltage of the first resistor; the second temperature sensor section being provided with a second temperature sensitive portion, a second resistor buried in the second temperature sensitive portion and having a positive resistance temperature coefficient, a second pair of current leads for feeding current to the second resistor and a second pair of voltage leads for detecting the voltage of the second resistor.